The present invention relates generally to the field of suction strainers, and more particularly to the field of suction strainers employed in the suppression pools of boiling water reactor (BWR) nuclear power plants.
A suction strainer employed in a suppression pool removes solids from a flow of liquid (e.g., water) being drawn into an emergency core cooling system (ECCS) pump. The flow of water is drawn through the suction strainer and then into the suction line of the ECCS pump. Employment of suction strainers is desirable because solid debris drawn into the suction line of a pump can degrade pump performance by accumulating in the pump or its suction or discharge lines, or by impinging upon and damaging internal pump components.
While almost any pump degradation can be characterized as being costly, the degradation of ECCS pump performance at BWR nuclear plants can be detrimental to safe plant shutdown following a loss of coolant accident (LOCA). At a BWR nuclear power plant following a LOCA, it is critical for the ECCS pumps to operate for an extended period of time in an undegraded fashion. In one mode of operation, the ECCS pumps are operated to recirculate water from the suppression pool back to the reactor core for the purpose of core cooling. A LOCA results from a high pressure pipe rupturing with such great force that large quantities of debris, such as pipe and vessel insulating material, and other solids, may be washed into the suppression pool. Conventional ECCS suction strainers currently installed in BWR plants would have a tendency to become clogged by such debris due to their small size and poor design. Also, when the large pressure pipes rupture with great force, suction strainers in the suppression pool are subjected to great hydrodynamic forces that can damage the suction strainers as well as subject the attachment recirculation piping to large reactive forces. These structural considerations, and space constraints, limit the size and shape of suction strainers in suppression pools.
Conventional BWR plant suction strainers are typically constructed and arranged in a manner such that, under full flow conditions, localized high entrance velocities are established through that portion of the suction strainer that is most proximate to the suction line of the pump, while low entrance velocities are established through that portion of the suction strainer that is more distant from the suction line of the pump. The high entrance velocities may draw more solid debris into contact with the suction strainer causing the portions of the suction strainer experiencing the high entrance velocities to experience higher head loss. As the portion of the suction strainer most proximate to the suction line collects debris, high entrance velocities are established at the portion of the suction strainer that is next closest to the suction line causing that portion to collect debris. This process often continues until the entire suction strainer has collected debris in varying quantities, resulting in a non-uniform build-up of debris on the outer surface of the strainer.
Localized high entrance velocities can be detrimental even when solids are not present in the liquid being pumped. For example, high entrance velocities can result in turbulent flow which tends to create greater pressure losses than laminar flow. Any such pressure losses reduce the net positive suction head (NPSH) available to a pump. As the NPSH available decreases, pump cavitation may occur. Similarly, localized high entrance velocities can cause vortexing. When a suction strainer is not sufficiently submerged, the vortexing can cause air ingestion which can severely degrade pump performance.
Attempts have been made to resolve certain of the problems associated with suction strainer-like devices in other applications. For example, cylindrical suction flow control pipes have been encircled with screen material and employed in water wells. Such wells typically employ a well pump above the ground surface and a riser pipe extending from the well pump to the water table. The suction flow control pipe is connected to the end of the riser pipe and extends further below the water table. Openings are defined through the side wall of the suction flow control pipe such that there is somewhat less open area near the riser pipe and somewhat more open area distant from the riser pipe. As a result, when water is drawn into the flow control pipe through the openings, a substantially uniform inflow distribution is defined along the length of the flow control pipe. While such suction flow control pipes offer some advantages, they are not suitable for all applications.
Attempts have been made, totally separate from flow control pipes, to increase filtering surface areas of BWR ECCS suction strainers in an effort to decrease pressure losses and thereby prevent pump cavitation. For example, such suction strainers may include a plurality of spaced, coaxial, stacked filtering disks. More particularly, such stacked disk suction strainers typically include an annular flange for attachment to the corresponding flange on the pump suction line. The stacked disk suction strainer provides an enhanced surface area and defines a longitudinal axis that is encircled by the attachment flange. A first disk is attached to the attachment flange. The first disk includes a pair of a radially extending, circular, disk walls, each of which encircle the longitudinal axis, and define a central hole. A first disk wall of the pair of disk walls is connected to the attachment flange. The first and second disk wall of the pair of disk walls face one another and are separated by a slight longitudinal distance. The first disk further includes an outer annular wall that encircles the longitudinal axis. The outer annular wall includes an annular first edge and an annular second edge. The entirety of the annular first edge of the outer annular wall is connected to the entire peripheral edge of the first perforated disk wall; and the entirety of the annular second edge of the outer annular wall is connected to the entire peripheral edge of the second perforated disk wall such that the pair of disk walls are connected at their periphery.
The stacked disk suction strainer further includes a plurality of inner annular walls that encircle the longitudinal axis, each of which includes an annular first edge and an annular second edge. The annular first edge of one of the inner annular walls is connected around the periphery of the central hole of the second disk wall. The annular second edge of that inner annular wall is connected around the periphery of the central hole of a disk wall of a second disk. The first and second disk walls, and the outer and inner annular walls are perforated and comprise the filtering surface of the stacked disk suction strainer. Additional perforated disks and inner annular walls are attached to one another in the above manner until the last disk is attached, wherein the outer disk wall of the last disk does not include a central hole. The stacked disk suction strainers may incorporate separate structural members to maintain the structural integrity of the stacked disk suction strainer. However, the conventional stacked disk suction strainers do not incorporate an internal core tube and related components, whereby the conventional stacked disk suction strainers are difficult to structurally reinforce and are susceptible to vortexing and the detrimental non-uniform localized entrance velocities discussed above.
There is, therefore, a need in the industry for an improved suction strainer.
Briefly described, the preferred embodiments of the of the present invention include a suction strainer that includes a filtering device with a strategically enlarged filtering surface and an internal core. The internal core is preferably in the form of an internal core tube, which is preferably an internal pipe with flow openings. In accordance with the preferred embodiments of the present invention, the internal core tube structurally reinforces the filtering device.
In accordance with the preferred embodiments of the present invention, the structural reinforcement provided by the internal core tube is enhanced by reinforcing structural members that extend radially from the internal core tube. The reinforcing structural members are preferably connected to and extend radially from and angularly around the internal core tube to structurally support the filtering surfaces of the external filtering structure. The internal core tube, in conjunction with the structural members, seeks to prevent air ingestion and vortexing. The suction strainer preferably extends away from the suction line of an ECCS pump to define a length, and in accordance with certain examples the preferred embodiments of the present invention, the internal core tube seeks to promote controlled inflow along the length to preclude the establishment of non-uniform localized entrance velocities through the filtering surface. In accordance with other examples of the preferred embodiments of the present invention, the internal core tube is not constructed to specifically promote such a uniform inflow along the length.
In accordance with the preferred embodiments of the present invention, the suction strainer is constructed in a manner that seeks to enlarge the filtering surface while minimizing the projected area of the suction strainer. The minimization of the projected area as well as structural reinforcement of the suction strainer enables the suction strainer to withstand high levels of hydrodynamic impact loading following a LOCA. The suction strainer also serves to minimize the bending moment and other reactive forces on the attachment ECCS piping in the BWR suppression pool.
In accordance with the preferred embodiments of the present invention, the filtering surface is defined by an external filtering structure that is attached to, extends from, and is built around the internal core tube and the reinforcing structural members. When the suction strainer is connected to the suction of a pump and submerged, a liquid flow path is established through the internal core tube and external filtering structure. The liquid originates exterior to the external filtering structure and is drawn through the filtering surfaces of the external filtering structure. The filtering surfaces separate solids from the liquid. The size of the filtering surface is enlarged by virtue of the fact that the filtering surface defines protrusions such that the distance that the filtering surface extends from the internal core tube alternates. The resulting enlarged filtering surface seeks to decrease average flow velocities through the filtering surface and thereby spread the collected solid debris in thinner layers, thereby decreasing overall pressure losses associated with the suction strainer. Once the liquid flows through the filtering surface, the liquid is drawn through the internal core tube and into the suction of the pump.
In accordance with the preferred embodiments of the present invention, the protrusions of the external filtering structure are in the form of a plurality of filtering plate assemblies that are connected to and extend radially from the internal core tube. Each plate assembly includes a pair of plate walls that face one another, define a distance therebetween, and are connected at their peripheries by an outer wall that surrounds the internal core tube. A separation distance is defined between neighboring plate assemblies. Inner walls connect between neighboring plate assemblies and extend around the internal core tube at a radius less than the radius of the outer walls. The outer and inner walls as well as the plate walls are perforated and comprise the filtering surfaces of the suction strainer. In accordance with first and second preferred embodiments of the present invention, the plurality of plate assemblies are preferably in the form of stacked disks that are spaced to defined troughs therebetween. In accordance with other embodiments, the plate assemblies are in other forms that increase the surface area of the suction strainer.
In accordance with preferred embodiments of the present invention, the internal core tube has a downstream end for connection to the pump suction flange and an upstream end distant from the downstream end. The internal core tube defines a longitudinal axis extending between the upstream and downstream ends. In accordance with the preferred embodiments of the present invention, a plurality of openings are defined through the side wall of the internal core tube. In accordance with certain examples of the preferred embodiments, the openings are constructed and arranged such that there is somewhat less open area near the downstream end than the upstream end, and the amount of open area tapers between the upstream end and the downstream end. As a result, when water flows into the internal core tube through the openings, a substantially uniform flow rate distribution is defined along substantially the entire length of the internal core tube.
It is therefore an object of the present invention to provide an improved BWR ECCS suction strainer.
Another object of the present invention is to increase safety by improving the operability of the ECCS of a BWR nuclear plant following a LOCA.
Yet another object of the present invention is to structurally reinforce a suction strainer sufficiently so that it can withstand the hydrodynamic forces following a LOCA in the suppression pool at a BWR nuclear plant.
Still another object of the present invention is to minimize reactive forces on the attachment ECCS piping following a LOCA.
Still another object of the present invention is to maximize the total strainer surface area within a limited geometric profile while providing a maximum strength strainer.
Still another object of the present invention is to simultaneously minimize both the thickness of collected debris on the strainer and the average entrance velocities to minimize the resultant NPSH of the ECCS following a LOCA.
Still another object of the present invention is to maximize the amount of time required to reach a particular head loss across the strainer.
Still another object of the present invention is to control the distribution of fluid flow over the strainer so as to collect debris uniformly, from disk to disk or from trough to trough, to allow scaling of the strainer for other flow rates with similar, but different size, strainers with different water flow rates.
Still another object of the present invention is to prevent vortexing and air ingestion.